In the third generation partnership project, 3GPP, there have been evaluations of both open loop beam forming and open loop antenna switching for uplink transmissions in WCDMA/HSPA. Both of these techniques are based on a User Equipment, a UE, with multiple transmit antennas which exploits the existing downlink feedback channels, e.g. F-DPCH, Fractional DPCH, or E-HICH, E-DCH HARQ Acknowledgement Indicator Channel, to determine a suitable pre-coding vector with which the transmitted signal is multiplied in an autonomous fashion in order to maximize the signal to noise plus interference ratio, SIR, at the receiving NodeB. A pre-coding vector comprises a set of pre-coding weights, i.e. a set of weight factors with which the signal from each of the antennas of the UE is multiplied with before transmission, in order to achieve a desired beam forming. The traffic from each antenna of the UE will be multiplied with one of the weight factors in a pre-coding vector prior to transmission, so that for a UE with N antennas we should have a pre coding vector [w1 . . . wN], where the signal from antenna 1 is multiplied by w1, the signal from antenna 2 is multiplied by w2, and where the signal from antenna N is multiplied by WN.
However, in such solutions, since the network is unaware of the pre-coding weights that the UE selects and applies, a receiving Node-B will experience a discontinuity in the measured power when a change in pre-coding weights occurs.
There have also been proposals in 3GPPP for introducing so called closed loop transmit diversity, by which we refer to both so called closed loop beam forming and closed loop antenna switching for WCDMA/HSPA. Contrary to the open loop techniques, where the UE decides the pre-coding weights in an autonomous way, in the closed loop techniques, the network, e.g., the serving Node-B determines the preferred pre-coding vector with which the signals from the antennas of the UE are to be multiplied. In order to signal the necessary downlink feedback information to the UE, the Node-B can either rely on one of the existing physical channels, e.g., F-DPCH, or a new downlink feedback channel could be introduced for this purpose.
Regardless of the physical channel that is used to signal the downlink feedback information, a key aspect of closed loop transmit diversity schemes is that the downlink feedback scheme should allow the downlink overhead to be minimized while, at the same time, ensuring that the UE receives downlink feedback with sufficient frequency and granularity in order for the multi-antenna transmissions to be beneficial.
The frequency and granularity of the downlink feedback that is required will vary with the following:                UE equipment (e.g., if the UE has antennas with different efficiency due to form factor considerations, or if there are significant performance differences between the antennas due to imperfections)        Small-scale shadowing (e.g., body effects), and        The coherence time of the wireless channel.        
The first two of the listed causes above will create variations that occur on a rather slow time scale and they can often be viewed as stationary for several seconds. The third cause, i.e. the wireless channel, may on the other hand vary on a much faster time scale. In fact, the speed with which the wireless channel varies will depend on environment, e.g. if the UE is stationary or mobile, if the UE is located indoors or outdoors, etc.
The term “effective channel” will be used here, sometimes also referred to as the “composite channel”, when referring to the radio channel that incorporates the effect of the transmit antenna(s), pre-coding weights (with which the transmitted signal is multiplied) as well as the wireless channel between the transmitting and receiving antenna(s).
Another term which will also be used in the following is the term “code book”, which will be used to refer to a pre-defined mapping by means of which the network, e.g. the NodeB and/or the RNC, can signal/convey information to the UE about the pre-coding vectors that the UE should apply. The codebook is composed of one or multiple code words, and each code word is used to inform the UE about a desired modification of the currently used pre-coded weight. This can also be expressed by saying that each code word in a code book identifies a pre-coding vector. It should be pointed out that the term “a desired modification” also refers to the case where it is desired to maintain the present pre-coding vector, i.e. a “zero change”.
In closed loop transmit diversity mode/beam forming described above, although the serving Node-B controls the pre-coding weight selection in the UE, in principle, it is also possible to let a non-serving Node-B be able to control the pre-coding weights used by the UE. One key issue in scenarios with multiple NodeBs, (for example, so called soft handover) is that even though the Node-B that controls the selection of pre-coding weights will know if and when a UE changes its pre-coding vector, the other Node-Bs in the so called active set will not be aware of this. From the perspective of these Node Bs, a change in pre-coding weights will result in a discontinuity in measured power, which is detrimental from a system performance point of view, since it will, for example, adversely affect load estimation.
In summary, closed loop transmit diversity/bean forming techniques will increase the coverage and capacity of the system, but will however be associated with additional traffic overhead, which stems from:                Additional uplink overhead since transmissions of multiple DPCCHs are needed in order to allow the Node-B to estimate the full effective channel. This is needed to determine suitable pre-coding weights.        Additional downlink overhead since the new feedback information carrying the pre-coding weight information needs to be signaled from the Node-B to the UE. As noted above, the intensity and granularity of this feedback information will, e.g., depend on how rapidly the wireless channel (and/or the effective channel) changes and the type of pre-coding weight selection algorithm implemented in the Node-B.        
Another drawback associated with closed loop transmit diversity is that too frequent and too large changes in pre-coding weights will cause abrupt changes in the power measured by neighbouring Node Bs which do not control pre-coding weight generation of the UE and which are therefore unaware of the change pre-coding weight. This will result in more varying interference levels, as well as inferior channel estimates. Clearly, this could be aided by limiting the rate and size with which the pre-coding weights are changed in these scenarios.